Gas storage vessels, such as gas cylinders or bottles, are commonly filled with gases by charging the gas into the vessel until the desired pressure is reached. It is desirable to fill the vessels as quickly as possible, but it is also important to accurately fill the vessels with the target quantity of gas. One problem that makes it difficult to accurately measure the amount of gas in a gas vessel is the temperature-pressure relationship of contained gases. By virtue of the gas laws, the pressure exerted by a given quantity of gas at constant volume is directly proportional to its temperature. Accordingly, as the temperature of a gas increases, so will the pressure of the gas. Thus, in filling gas storage vessels by pressure measurements it is important that the gas in the vessel when it approaches its "filled" condition be at a preselected temperature to ensure that the correct amount of gas is charged into each vessel being filled.
Since it is desirable to fill the gas vessel in the shortest time period, it is customary to immediately open the fill valve to the wide-open position. This causes an immediate blast of gas to enter the empty vessel, which causes the temperature of the gas being charged into the vessel to rise rapidly as it impinges the walls of the vessel. Rapid filling of the vessel does not continue to cause a rapid temperature increase throughout the filling process, and the initially heated gas cools as additional gas is charged into the vessel. But often the gas temperature does not return to the preselected temperature during the filling process, so that it is difficult or impossible to charge the correct amount of gas into the vessel without the time-consuming step of cooling the gas to the preselected temperature.
The problem is exacerbated when filling the gas vessel with mixtures of gases. In this case, a first gas is charged into the vessel until the vessel contains the desired amount of this gas, and then a second gas is charged into the vessel until the desired amount of that gas is charged into the vessel. This procedure is repeated until all of the gases are charged into the vessel. Since it is generally necessary that the composition of the gas mixture meet narrow specifications. It is important that the temperature of the gas mixture be within narrow limits as the fill endpoint for each component is approached. Thus, if excessive heating occurs during the early stages of the filling process, the gas must be cooled before making the final measurement of the first component, and perhaps cooling may have to be done before final measurement of each component of the gas mixture.
Another phenomenon which makes it difficult to accurately fill gas vessels with specific quantities of gases is that the pressure drop in a gas pipeline through which a gas is flowing is directly proportional to the velocity of the gas flowing through the pipeline: the pressure drop over a given length of pipeline increases as the rate of flow of gas through the pipeline increases. Thus, if a gas vessel is being filled with gas from a gas line which has a pressure gauge upstream of the gas vessel, the pressure at the pressure gauge will be higher than the actual pressure in the vessel, so that if gas is flowing through the fill line at a rapid rate and the pressure gauge is used to determine the cutoff point of the fill process, the gas vessel will not be filled with the correct quantity of gas.
Since time is valuable and accuracy of filling is important, it would be desirable to fill empty gas vessels with single gases or gas mixtures by a method which does not cause a rapid rise of the temperature of the gas when gas is introduced into an empty vessel, and to eliminate or minimize the error caused by differences in pressure at a pressure gauge in the fill line and the actual pressure in the vessel. The present invention provides a method which meets these objectives.